ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

Stress waves contributions to stone fragmentation during lithotripsy are
investigated both theoretically and experimentally. A two-dimensional finite
difference scheme is developed to analyze the time evolution of the strain
fields inside irregularly shaped solids subjected to ultrasonic pulses that
simulate lithotripter shock waves. The reflections and superposition of stress
waves inside the stones are analyzed to better understand the effects of stone
parameters and geometry on the induced internal strains and fragmentation during
lithotripsy. Numerical results show the focusing effect of the concave
backsurface of a spherical stone, with the subsequent formation of focal zones
(caustics). The focusing is reduced when a section of the back surface of the
stone is removed. Principal strain contours depict the time evolution of the
stress waves as they refract and reflect at the stone boundaries. Locations of
maximum stresses are calculated and compared to locations of crack initiation in
experiments with stones of similar geometry. The calculated time evolution of
strain at fixed points within a stone is compared to imbedded silicon strain
gauge measurements. Fracture characteristics of synthetic stones show internal
crack initiation and subsequent propagation to external stone surfaces,
indicative of internal stress fragmentation mechanisms. [Work supported by NIH.]